I. Interference in Wireless Networks
Inter-cell interference can be experienced by a wireless communication device (WCD) such as a user equipment (UE) on the downlink and by a base station on the uplink. To address this interference, Inter-cell Interference Coordination (TCIC), Enhanced ICIC (eCIC) and further eICIC (FeICIC) techniques have been developed in 3GPP. The eICIC and FeICIC are time domain schemes that enable interference mitigation by the virtue of resource partitioning in the time domain between an aggressor node that is a source of the interference and a victim node that receives the interference. These schemes partly or fully mitigate the interference experienced by a WCD being served by the victim node or, more specifically, by the receiver of the WCD, which may receive both a desired signal from the victim node and an interfering signal from the aggressor node.
In the time domain eICIC or FeICIC schemes, the subframe utilization across different cells is coordinated in time through backhaul signaling (e.g., over the X2 interface between the base station of two cells). The subframe utilization is expressed in terms of a time domain pattern of low interference subframes or “low interference transmit pattern.” Examples include Almost Blank Subframe (ABS) patterns. The Almost Blank Subframes (ABSs) are configured in an aggressor node (e.g., a macro node) and are used to protect resources in subframes in a victim node (e.g., a pico node) receiving strong inter-cell interference. A base station that is serving a UE or other WCD can signal one or more measurement patterns to inform the UE about the resources or subframes which the UE should use for performing measurements on a target victim node (e.g., the serving pico node and/or neighbouring pico nodes).
The schemes are also applicable to other specific types of deployment scenarios such as CoMP, multipoint operation, multiflow operation, multi-carrier deployment, or any combination of these scenarios. Such scenarios may exist in both homogeneous and heterogeneous networks.
II. Types of Wireless Networks
A. Homogeneous Network
A homogeneous network is a 1-tier system comprising a single layer of radio network nodes (e.g., all nodes that are high power nodes (HPNs) such as wide area base stations serving macro cells). In another example, a homogeneous network may also comprise only low power nodes (LPNs) (e.g., local area base stations serving pico cells). When different cells of a homogenous network are under similar levels of load, a user equipment (UE) typically receives equally strong signals from the serving node (which may also be a measured node) and from the closest neighboring nodes, especially when the UE is located in the border region between the two cells. Therefore, in a homogeneous network, resource partitioning between serving and neighboring cells for the purpose of inter-cell interference mitigation is not as critical as in a heterogeneous network.
B. Heterogeneous Network
The heterogeneous network comprises 2 or more layers where each layer is served by one type of base station (BS) class or type. An example of a heterogeneous network includes nodes of different power classes, such as a set of high power nodes and low power nodes in a geographical region. A BS power class is defined in terms of maximum output power and other radio requirements (e.g., frequency error etc.) which depend upon the maximum output power. The maximum output power, Pmax, of the base station is the mean power level per carrier measured at the antenna connector in a specified reference condition. The rated output power, PRAT, of the BS for different BS power classes is expressed in Table 0.
TABLE 0Base Station rated output power in LTE (FDD and TDD)BS classPRATWide Area BS- (see note)Medium Range<+38 dBmBSLocal Area BS ≤+24 dBmHome BS≤+20 dBm (for one transmit antenna port)≤+17 dBm (for two transmit antenna ports)≤+14 dBm (for four transmit antenna ports)<+11 dBm (for eight transmit antenna ports)NOTE:There is no upper limit for the rated output power of the Wide Area Base Station.
A heterogeneous network, such as cells in a co-channel scenario, brings more challenges in terms of managing interference. The interference can be addressed using the ICIC, eICIC, and FeICIC techniques described above.
II. Interference Mitigation
Some nodes include an advanced receiver (also called an enhanced receiver) for performing interference mitigation. Examples of the advanced receiver include an interference mitigation receiver, interference cancellation receiver, interference suppression receiver, interference rejection receiver, interference aware receiver, and interference avoidance receiver, and some of these terms are used interchangeably. Interference cancellation or suppression by such advanced receivers can lead to the elimination of the interference, in which case the interference is completely cancelled, whereas in other cases the impact of interference on the useful signal is reduced. The advanced receiver can be used at a wireless communication device and/or at a radio network node (e.g. a BS, a relay, etc.) for improving the reception of the received, wanted radio signals.
A well-known example of an advanced receiver is a Minimum Mean Square Error Interference Rejection Combining (MMSE-IRC) receiver. An example of a more sophisticated advanced receiver is the Minimum Mean Square Error-turbo Interference Cancellation (MMSE-turbo IC) receiver, which is capable of performing non-linear subtractive-type interference cancellation, which can be used to further enhance system performance. Even the use of multiple receive antennas at a receiver can be considered an advanced receiver.
III. Mechanisms to Trigger Interference Mitigation of Physical Signals
For co-channel heterogeneous network deployment in release 11, a large cell range expansion (CRE) of up to 9 dB is supported. When a UE is in the CRE region of a low power node (LPN), the received signal at the UE can be interfered by, e.g., up to 2 strong macro aggressor nodes. In this scenario the received signal to interference plus noise ratio (SINR) (e.g., SCH Ês/Iot or CRS Ês/Iot) at the UE when the UE is served by a LPN and is located in the CRE region of the serving cell can be very low (e.g., down to −11 dB). In order to correctly detect received signals, the UE in the CRE region has to mitigate (e.g., reduce or cancel) interference on certain physical signals (e.g., on a CRS signal).
To facilitate UE interference mitigation of these physical signals, a radio network node can assist the UE by providing to the UE assistance information, such as a list of assistance information (e.g., physCellID, antennaPortsCount, mbsfn-SubframeConfigList) as specified in TS 36.331. When a physical signal, such as a received CRS, carries assistance information that is for a cell with CRS colliding with that of the CRS of the cell being measured or to be measured, the UE may use the CRS assistance information to mitigate CRS interference on the subframes indicated by the following parameters specified in TS 36.331: measSubframePatternPCell, measSubframePatternConfigNeigh and csi-MeasSubframeSet1.
As another example, the CRS assistance information contains a list of aggressor cells, their antenna port information, and also their multi-broadcast single-frequency network (MBSFN) configuration.
It has also been specified in TS 36.133 v11.2.0 that the UE shall meet the measurement requirements when the UE is provided with CRS assistance information, which is valid over the measurement period.
The reception of information from physical signals (e.g., CRS assistance information) at the UE is used by the UE to perform the interference cancellation on physical signals (e.g., CRS, etc.). However, in heterogeneous network deployment, the UE typically applies interference cancellation on restricted subframes indicated in measurement patterns, which are signaled to the UE by the serving radio node via RRC protocol.